1. Field of the Invention
The present invention relates to a motor driving apparatus and a control method thereof, and more particularly, to a brushless DC motor.
2. Description of Related Art
Permanent magnet synchronous motors having a small size and a high output are now widely employed in the field of consumer products such as washing machines, refrigerators, and air conditioners which are required to be downsized.
Recent technological innovations in the field of power devices such as MOS-FETs make it possible to realize an inverter control in which a commercial AC power supply is temporarily rectified and converted into a DC power supply and is then regenerated into a given drive waveform through a switching operation of a power device. The inverter control facilitates power saving and control operation. Further, brushless DC motors for driving the permanent magnet synchronous motor as described above are now widely used.
FIG. 6 shows an inverter circuit 1A of a typical brushless DC motor. As shown in FIG. 6, the inverter circuit 1A includes transistors Q1 to Q6. The transistors Q1 and Q2, the transistors Q3 and Q4, and the transistors Q5 and Q6 are connected in series between a DC power supply voltage VDD and a ground voltage GND. The transistors Q1 to Q6 receive control signals U+, U−, V+, V−, W+, and W−, respectively.
FIG. 7 shows an example of voltage operation waveforms of the control signals U+, U−, V+, V−, W+, and W−. The transistors Q1 to Q6 each perform a switching operation of repeatedly turning on and off in pulse shapes as shown in FIG. 7. For example, during a time period between a time t0 and a time t2, the control signals U+ and V− become the high level at the same time, and the transistors Q1 and Q4 are simultaneously turned on. As a result, a current flows through a U-phase coil and a V-phase coil of a brushless DC motor 2A. Likewise, during a time period between the time t2 and a time t4, the control signals V+ and W− become the high level at the same time, and the transistors Q3 and Q6 are simultaneously turned on. As a result, a current flows through the V-phase coil and a W-phase coil of the brushless DC motor 2A. After that, the transistors are switched in response to the control signals in a similar manner, which enables the inverter circuit 1A to generate a drive current for the brushless DC motor 2A.
In this exemplary embodiment, the brushless DC motor 2A is a three-phase motor. Thus, the on/off timing of each of the transistors Q1 of Q6 is adjusted so that currents having phases shifted by 120° from each other flow through the U-phase, V-phase, and W-phase coils of the brushless DC motor 2A. Note that the control signals U−, V−, and W− are inverted signals of the control signals U+, W+, and V+, respectively.
Furthermore, pulse width modulation (hereinafter, referred to as “PWM”) control is employed to control driving of a motor through the switching operation as described above. The PWM control is now most widely employed as a method of controlling a DC motor. The PWM control will be briefly described with reference to graphs shown in the upper and lower portions of FIG. 8. Note that the graph in the lower portion of FIG. 8 shows the control signal U+, for example, as one phase of the control signals U+, U−, V+, V−, W+, and W− each having a modulated pulse width. The other control signals have a shifted phase, but the control signals are signals having a similar waveform or inverted signals thereof.
In the PWM control of this exemplary embodiment, a triangular wave is used as a carrier as shown in the graph of the upper portion of FIG. 8. In addition, a command voltage signal shown in the graph of the upper portion of FIG. 8 is used to control the number of rotations of the motor to a desired value. The command voltage signal and the triangular wave are compared with each other to thereby determine the pulse width of the control signal U+ as shown in the graph of the lower portion of FIG. 8.
As shown in the graph of the lower portion of FIG. 8, when the amplitude of the command voltage signal is high, the pulse width of the control signal U+ is large. Meanwhile, when the amplitude of the command voltage signal is low, the pulse width of the control signal U+ is small. When the pulse width is large, the transistor is turned on for a long time, which leads to an increase in the current flowing through the coils of the motor and an increase in the number of rotations of the motor. Meanwhile, when the pulse width is small, the transistor is turned on for a short period of time, which leads to a reduction in the number of rotations of the motor. In the PWM control, as described above, the pulse width modulation is performed on the command voltage signal, and the number of rotations of the motor is controlled by, for example, the control signal U+ having the modulated pulse width.
In this case, in the inverter control for the brushless DC motor, if the driving of the motor cannot be controlled due to rapid deceleration of the motor, a system malfunction, or the like, the motor comes into a regenerative (power generation) state owing to load inertia, resulting in generation of a large back electromotive force. In order to prevent the motor, the transistors of the inverter circuit, a smoothing capacitor of a converter circuit for supplying a power supply to the inverter circuit, or the like from being damaged by the back electromotive force, it is necessary to provide means for removing the back electromotive force.
Japanese Unexamined Patent Application Publication No. 02-290174 discloses a motor driving apparatus 1B for short-circuiting the coils of the motor or short-circuiting the coils and a ground, upon detection of an abnormality of a power supply supplied to a motor drive circuit by the back electromotive force. FIG. 9 shows a circuit configuration of the motor driving apparatus 1B disclosed in Japanese Unexamined Patent Application Publication No. 02-290174. As shown in FIG. 9, the motor driving apparatus 1B includes a power supply circuit 2B, a motor drive circuit 3B, a motor 4B, a detection circuit 5B, a bias circuit 6B, and an FET 7B.